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Employee Benefit Plans (Fair Value of Investments) (Details) - CAD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 100.00% | 100.00% |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 2,300 | $ 2,408 |
Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 796 | 1,048 |
Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 234 | $ 132 |
Cash and cash equivalents [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 1.00% | 1.00% |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 30 | $ 32 |
Cash and cash equivalents [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 30 | 32 |
Cash and cash equivalents [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 0 | $ 0 |
Net In Transit [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | (2.00%) | (1.00%) |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ (56) | $ (36) |
Net In Transit [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | (56) | (36) |
Net In Transit [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 0 | 0 |
Net Asset Value Member [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 1,270 | $ 1,228 |
Open Ended Invesments Measured at NAV [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 36.00% | 34.00% |
Defined Benefit Plan, Fair Value of Plan Assets | [1] | $ 820 | $ 819 |
Open Ended Invesments Measured at NAV [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 0 | 0 |
Open Ended Invesments Measured at NAV [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 0 | $ 0 |
Collective Trust Measured at NAV [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 19.00% | 18.00% |
Defined Benefit Plan, Fair Value of Plan Assets | [2] | $ 450 | $ 409 |
Collective Trust Measured at NAV [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 0 | 0 |
Collective Trust Measured at NAV [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 0 | $ 0 |
Equity Securities, Canadian [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 8.00% | 9.00% |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 191 | $ 214 |
Equity Securities, Canadian [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 191 | 214 |
Equity Securities, Canadian [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 0 | $ 0 |
Equity Securities, United States [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 14.00% | 16.00% |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 330 | $ 390 |
Equity Securities, United States [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 330 | 390 |
Equity Securities, United States [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 0 | $ 0 |
Equity Securities, Other [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 7.00% | 8.00% |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 157 | $ 197 |
Equity Securities, Other [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 157 | 197 |
Equity Securities, Other [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 0 | $ 0 |
Government Fixed Income Securities [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 5.00% | 3.00% |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 119 | $ 72 |
Government Fixed Income Securities [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 0 | 0 |
Government Fixed Income Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 119 | $ 72 |
Corporate Fixed Income Securities [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 5.00% | 2.00% |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 108 | $ 56 |
Corporate Fixed Income Securities [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 0 | 0 |
Corporate Fixed Income Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 108 | $ 56 |
Other Fixed Income Securities [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 7 | $ 5 |
Other Fixed Income Securities [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 4 | 5 |
Other Fixed Income Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 3 | $ 0 |
Mutual Funds [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 6.00% | 10.00% |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 132 | $ 246 |
Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 132 | 246 |
Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 0 | $ 0 |
Other Investments [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Actual Plan Asset Allocations |  | 1.00% |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 12 | $ 4 |
Other Investments [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 8 | 0 |
Other Investments [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 4 | 4 |
Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 2,300 | 2,408 | $ 2,208
Postretirement Benefit Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 49 | $ 45 | $ 39
[1] | NAV investments are open-ended registered and non-registered mutual funds, collective investment trusts, or pooled funds. NAV’s are calculated daily and the funds honor subscription and redemption activity regularly.
[2] | The common collective trusts are private funds valued at NAV. The NAVs are calculated based on bid prices of the underlying securities. Since the prices are not published to external sources, NAV is used as a practical expedient. Certain funds invest primarily in equity securities of domestic and foreign issuers while others invest in long duration U.S. investment grade fixed income assets and seeks to increase return through active management of interest rate and credit risks. The funds honor subscription and redemption activity regularly.

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